Satellite communication systems must transmit signals vast distances from earth to satellites in orbit and vice versa. Additionally, satellites have strict power consumption limits that require the communication systems to operate at very high efficiencies of both power use and usage of available communication bandwidth. Many satellites use high power amplifiers (HPAs) for communication purposes. Typically, HPAs operate most efficiently at (or near) saturation. Unfortunately, operation of HPAs at (or near) saturation can lead to inter-symbol interference (ISI) in output channels. The output of a transmitter can be seen as a sequence of symbols called a phrase. Each symbol represents a sequence of bits, in the case of 8PSK, each symbol represents 3 bits. A transmitter will output the phrase one symbol at a time during transmission. As a transmitter shifts from one symbol to the next in the phrase, previous and future output symbols may cause interference in the output of the current symbol. This interference in the current symbol caused by past and future symbols is ISI.
In current satellite communications systems, the uplinked signal is amplified and channelized in a transparent satellite transponder. Further, to satisfy the aggressive demand for higher satellite throughput, industry trend is moving towards sharing the transponder amplifier by multiple carriers, each employing high-order modulations that are spectrally compact. Joint amplification of multiple-carrier signals using a common HPA is also employed to meet satellite power and mass requirements. Given the inherently nonlinear nature of the amplifier when driven efficiently closer to saturation, however, the amplification of multiple carriers through a common HPA further exacerbates the issue of intermodulation distortion that needs to be appropriately compensated. The non-linear effects are even more prominent in this scenario due to onset of such intermodulation products causing adjacent channel interference (ACI). A significant guard-band between the carriers may be needed in order to avoid ACI, thereby reducing spectral efficiency. Additionally, use of multiple carriers leads to high peak to average power ratios, which increases the back-off leading an amplification efficiency loss. Moreover, on-board channelization filters (IMUX/OMUX) introduce inter-symbol interference (ISI), which further degrades the performance. In order to mitigate the nonlinear distortion, significant back off is required, leading to power efficiency loss.
Improved technological solutions are needed to satisfy the aggressive demand for satellite throughput in broadband and broadcasting applications. The key is to maximize system efficiency at many levels, such as payload mass efficiency through sharing each satellite HPA (i.e., travelling-wave tube amplifier (TWTA)) by multiple carriers, bandwidth efficiency by employing frequency-compact carriers with high-order modulations using Amplitude and Phase Shift Keying (APSK) that are tightly packed in the spectrum, and power efficiency by properly operating the TWTA close to saturation. Due to the inherently nonlinear nature of TWTAs, this creates an environment with substantial nonlinear distortion that is significantly detrimental to system performance if left uncompensated.
Towards the goal of mitigating the nonlinear distortion, the articles (1) B. F. Beidas and R. I. Seshadri, “Analysis and compensation for nonlinear interference of two high-order modulation carriers over satellite link,” IEEE Trans. Comm., vol. 58, no. 6, pp. 1824-1833, June 2010, and (2) B. F. Beidas, “Intermodulation distortion in multicarrier satellite systems: Analysis and turbo volterra equalization,” IEEE Trans. Comm., vol. 59, no. 6, pp. 1580-1590, June 2011 provide an analytical framework that characterizes intermodulation distortion (IMD) resulting from such scenarios. The article (2) investigates receiver algorithms based on Turbo Volterra processing, where soft information between decoders and multicarrier Volterra equalizer is exchanged to progressively improve performance. Further, the articles (3) X. Wang and H. V. Poor, “Iterative (Turbo) soft interference cancellation and decoding for coded CDMA,” IEEE Trans. Comm., vol. 47, no. 7, pp. 1046-1061, July 1999, (4) H. El-Gamal and E. Geraniotis, “Iterative multiuser detection for coded CDMA in AWGN and fading channels,” IEEE J. Select. Areas Comm., vol. 18, no. 1, pp. 30-41, January 2000, and (5) B. F. Beidas, H. El-Gamal, and S. Kay, “Iterative interference cancellation for high spectral efficiency satellite communications,” IEEE Trans. Comm., vol. 50, no. 1, pp. 31-36, January 2002 discuss the technology of multi-user detection (MUD) in nonlinear systems.
According to other current approaches, the articles (7) G. Karam and H. Sari, “Analysis of predistortion, equalization, and ISI cancellation techniques in digital radio systems with nonlinear transmit amplifiers,” IEEE Trans. Comm., vol. 37, no. 12, pp. 1245-1253, December 1989, (8) L. Giugno, M. Luise, and V. Lottici, “Adaptive pre- and post-compensation of nonlinear distortions for high-level data modulations,” IEEE Trans. Wireless Comm., vol. 3, no. 5, pp. 1490-1495, September 2004, and (9) R. De Gaudenzi, A. G. Fabregas, and A. Martinezi, “Performance analysis of turbo-coded APSK modulations over nonlinear satellite channels,” IEEE Trans. Wireless Comm., vol. 5, no. 9, pp. 2396-2407, September 2006 discuss static or memoryless data predistortion techniques that simplify attempts to correct for the constellation warping, without mitigating the nonlinear intersymbol interference (ISI). Also, the article (10) E. Biglieri, S. Barberis, and M. Catena, “Analysis and compensation of nonlinearities in digital transmission systems,” IEEE J. Select. Areas Comm., vol. 6, no. 1, pp. 42-51, January 1988 proposes compensation based on pth order Volterra inverse of the nonlinearity, and article (11) C. Eun and E. Powers, “A new volterra predistorter based on the indirect learning architecture,” IEEE Trans. Signal Process., vol. 45, no. 1, pp. 223-227, January 1997 introduces an adaptive version of the Volterra inverse. Further, predistortion based on nonlinear polynomial models is investigated in article (12) L. Ding, G. Zhou, D. Morgan, M. Zhengxiang, J. Kenney, K. Jaehyeong, and C. Giardina, “A robust digital baseband predistorter constructed using memory polynomials,” IEEE Trans. Comm., vol. 52, no. 1, pp. 159-165, January 2004. Dynamic data predistortion (based on look-up tables), generated by minimizing mean-squared error (MSE), is investigated in the articles (13) G. Karam and H. Sari, “A data predistortion technique with memory for QAM radio systems,” IEEE Trans. Comm., vol. 39, no. 2, pp. 336-344, February 1991, and (14) E. Casini, R. De Gaudenzi, and A. Ginesi, “DVB-S2 modem algorithms design and performance over typical satellite channels,” Int'l J. on Satellite Comm. and Networking, vol. 22, no. 2, pp. 281-318, 2004—however, the size of LUTs grows exponentially with the modulation order and the memory span of the channel, both of which are large in efficient satellite systems. To avoid the use of LUTs, successive application of distortion cancellation is proposed in U.S. Pat. No. 8,355,462, “System and method for combined predistortion and interference cancellation in a satellite communications system.” With respect to multicarrier systems (where multiple carriers share the same nonlinearity), article (16) R. Piazza, M. R. Bhavani Shankar, E. Zenteno, D. Ronnow, J. Grotz, F. Zimmer, M. Grasslin, F. Heckmann, and S. Cioni, “Multicarrier digital predistortion/equalization techniques for nonlinear satellite channels,” Proc. 30th AIAA Int'l Comm. Satellite System Conference (ICSSC), September 2012 proposes a multicarrier PD is proposed, which is based on modifying the transmitted symbols by a 3rd-order Volterra inverse of the nonlinearity and simplified using the memory polynomial approach of Reference (12). Avoiding the use of LUTs is particularly important in the multicarrier case as the size of LUTs grows exponentially not only with modulation order and memory span of the channel but also with the number of carriers.
As bandwidth demands continue to increase, system requirements are driven to new levels, such as requirements for further increased satellite throughput, improved spectral efficiency, further satellite power and mass optimization and further improved decoding capabilities of system terminal receivers (without driving the receiver complexity and processing burdens to inefficient and impractical levels). What is needed, therefore, are approaches that provide improved compensation for mitigating nonlinear distortion in such high throughput multicarrier satellite communications systems (employing HPAs that process composite multicarrier signals for transmission over respective satellite channels, where the HPAs operate at or near saturation), which can be deployed entirely at the transmitter or gateway.